Project Abstract Intravenously injected nanoparticulate drugs are cleared by phagocytes and elicit immune reactions, including cytokine release and pseudoallergy (up to 10% in Doxil and 30% in Onpattro). There is substantial preclinical evidence on the involvement of the serum complement system in these responses. Understanding the mechanisms of complement activation in patients and designing preventive strategies can improve the safety and efficiency of nanoparticle-based drugs. We found that cell membrane-derived inhibitors of complement can effectively shut off complement activation and prevent the uptake of nanoparticles by leukocytes in the blood of healthy donors. Furthermore, the inhibitors that are designed to target complement deposits on the nanoparticle surface showed picomolar activity, for all nanoformulations we tested. The available data suggest a novel hypothesis wherein less than 1% of nanoparticles act as initiators of the complement cascade. Our long-term goal is to conduct a clinical trial of combination of nanomedicines with complement inhibitors. Such a trial will provide the nanomedicine field with the ultimate answers on the role of complement in hemocompatibility, clearance, and infusion reactions in humans. The main objectives of this proposal are 1) to further understand mechanisms of complement initiation by nanoparticles; 2) to design a rapid companion test to measure complement activation by nanoparticle-based drugs; 3) to study the efficacy of nanoparticle-binding inhibitors in blood of defined cohorts of patients in vitro and in dogs. This research will shift the existing paradigm of stealth design: it will enable the control and fine- tuning of the biocompatibility of nano-sized drug delivery and imaging systems using specific inhibitors.